The present invention relates to an automatic gain control circuit.
FIG. 1 shows an automatic gain control circuit used in the intermediate frequency amplifier stage of an AM radio.
In the figure, a resistor R1 and diodes D1 and D2 are connected in series between a power source V.sub.cc and ground. A junction A between the resistor R1 and the diode D1 provides a constant voltage. The bias voltage at the junction A is applied to the base of a transistor TR1 through a resistor R2 for supplying a current to the base of the transistor TR1. An input signal source V.sub.g is coupled with the base of the transistor TR1 through a capacitor C1. The transistor TR1 is connected at the emitter to ground through a resistor R3 and at the collector to the power source V.sub.cc via a resistor R4.
The collector TR1 is connected at the collector to an amplifier circuit 1 by way of a capacitor C2. The transistor TR1 operates as an emitter-common voltage amplifier circuit with a gain R4/(r.sub.e =R3). The r.sub.e is an emitter resistance of a transistor TR1. The output signal of the amplifier circuit 1 is applied to a detector circuit 2 of which the output signal is applied to the base of a transistor TR2 via a rectifier circuit 3 and is outputted to exterior. The transistor TR2 is connected at the emitter to ground and at the collector to the base of the transistor TR1.
In this circuit, when the input signal, i.e. the voltage to be applied to the base of the transistor TR1, rises, the output signal voltage of the detector circuit 2 rises and the voltage to be applied to the base of the transistor TR2 via the rectifier circuit 3 also rises. As a result, the transistor TR2 is conductive thereby to increase the voltage drop across the resistor R2. Consequently, the bias voltage of the transistor TR1 decreases and the emitter resistance r.sub.e becomes large and the voltage gain of the transistor TR1 is reduced, leading to decrease of the output voltage of the detector circuit 2.
In short, this automatic gain control circuit performs an automatic gain control by using a non-linear characteristic of the emitter resistor r.sub.e of the transistor TR1 with respect to variation of the base voltage of the same. When the input signal is small, i.e. r.sub.e &lt;R3, the non-linearity of the emitter resistor r.sub.e little influences the output signal and thus the distortion of the output signal is small. On the other hand, when the input signal voltage is large, i.e. r.sub.e &gt;R3, the non-linearity of the emitter resistance r.sub.e greatly influences the output signal, resulting in a large amount of distortion of the output signal.